The present disclosure relates to a controller, a controlling method, and a solar cell. More particularly, the disclosure relates to a controller and a controlling method each of which enables deterioration of power generation characteristics of a solar cell to be suppressed, and the solar cell.
A solar cell as a photoelectric conversion element for converting a solar light into an electrical energy uses the solar light as an energy source. Therefore, an influence of the solar cell exerted on the earth's environment is very small. Thus, in recent years, further popularization of the solar cell has been expected.
Heretofore, a crystalline silicon system solar cell using single-crystalline or polycrystalline silicon, and an amorphous silicon system solar cell have been mainly used as the solar cell.
On the other hand, a dye sensitized solar cell which was proposed in 1991 by Michael Grätzel et al. has attracted attention because a high photoelectric conversion efficiency can be obtained in the dye sensitized solar cell, unlike the existing silicon system solar cell, a large-scale system is not necessary during the manufacture for the dye sensitized solar cell, the dye sensitized solar cell can be manufactured at low cost, and so forth. The dye sensitized solar cell, for example, is described in Nature, 353, p. 737 (1991).
Now, the dye sensitized solar cell uses an electrolyte layer (in a phase of either a liquid or a solid), including an organic solvent or an ion liquid containing therein a redox species, a gel and the like. Therefore, in the case where although the dye sensitized solar cell is put under a light radiation environment (under a power generation environment), an external circuit connected between power collecting portions of the dye sensitized solar cell is held in an open circuit state, or the dye sensitized solar cell is under a use circumstance in which an electric power is not consumed so much (in a word, in the case where a release state is provided across the power collecting portions), a polarization phenomenon is easy to generate in the electrolyte layer of the dye sensitized solar cell. Also, it is feared that when the generation of such a polarization phenomenon causes a reduction in the power generation characteristics of the dye sensitized solar cell.
Describing more specifically, it is known that in the case where the light is radiated to the dye sensitized solar cell, and the external circuit of the dye sensitized solar cell is held in the open circuit state, the electrons are accumulated in an interface between a conductive transparent electrode (made of an FTO (Fluorine Tin Oxide), an ITO (Indium Tin Oxide) or the like) of a semiconductor electrode and TiO2.
Since not only under the solar light, but also under an indoor fluorescent light, the dye becoming an electron supply source is excited, the electrons are continuously supplied from the dye, and thus the electron accumulation can be caused.
The polarization is generated inside the dye sensitized solar cell when the electrons are continuously accumulated in the interface between the conductive transparent electrode and TiO2 in such a way. Specifically, the electrons are leaked from the interface between the conductive transparent electrode and TiO2, and a redox component of the electrolyte layer is reduced, so that composition balance between an oxidant and a reductant is lost.
With regard to the possibility of the reverse electron transfer, for example, there are considered three kinds of reactions: (1) deactivation of the dye from the excited state; (2) movement of the electrons from TiO2 to the redox component; and (3) movement of the electrons from the conductive transparent electrode to the redox component. Of the three kinds of reactions, the reaction mentioned in (3) is fastest. Therefore, the reaction stated in the paragraph right before this paragraph is easy to preferentially occur, and thus the polarization phenomenon is easily generated.
In particular, since the larger an area of the dye sensitized solar cell (the larger the current generated by the dye sensitized solar cell), the more an amount of electrons generated in one dye excitation, the polarization phenomenon is readily generated.
Such a polarization phenomenon is the phenomenon peculiar to the dye sensitized solar cell using the electrolyte layer (in the phase of either the liquid or the solid), including the organic solvent or the ion liquid containing therein the redox species, the gel and the like, and is also a deterioration mode which could not be supposed in the existing silicon system solar cell (using the single-crystalline silicon, the amorphous silicon or the like).
In order to cope with such a situation, there was devised a method in which an electrode for current application was specially provided in the dye sensitized solar cell, a reverse current was applied from the electrode to the dye sensitized solar cell by using an external power source to reduce the polarization phenomenon generated, thereby recovering the deteriorated power generation characteristics of the dye sensitized solar cell. This method, for example, is disclosed in Japanese Patent Laid-Open No. 2008-192441 (Patent Document 1).